In prior art location tracking systems, a tracking device provides its respective location, e.g., its latitude and longitude, or information from which such location can be determined, to a remote monitoring center. At the center, or some other associated place, the location of the tracking device is determined, if necessary, and then stored and/or processed. To this end, each tracking device receives signals from GPS satellites and/or wireless signals from terrestrial antennas, hereinafter “other wireless signals”. Each tracking device is typically carried by an entity, hereinafter the “monitored entity”, and there may be many different types of monitored entities, including but not limited to, an individual, a moving vehicle, a product, or a product container. The information stored at the remote monitoring center or some other associated location may be used to provide a history of the location of the tracking device and its associated entity as a function of time.
Each tracking device can be implemented as a unitary device, the so-called “one-piece” tracking device, or as multiple devices that communicate with one another. In either case, each tracking device contains a GPS and/or other wireless signal receiver for respectively receiving GPS signals from satellites or receiving other wireless signals. The signals received from such sources may be used to determine the location of the tracking device, such tracking device location determination being either within the device and/or at a remote location. A “dumb” tracking device is one that merely retransmits the received GPS or other wireless signal to a remote location wherein the location of the tracking device is derived from the received signals. A “smart” tracking device, on the other hand, possesses the capability of deriving its location from the received GPS or other wireless signals and subsequently transmits its determined location to a remote location. Such transmissions to the remote location are typically periodic to reduce consumption of the tracking device's internal battery, but can be immediate, if desired or if one or more prescribed “alarm” conditions are detected. Alarm conditions include, but are not limited to, detection of tracking device tampering, or a determination that the device is located in a prohibited zone, i.e., an “exclusion zone” or that the device is outside of a permitted zone, i.e., a “inclusion zone”. Such zones can be set individually to match the requirements for the monitored entity. Smart or dumb tracking devices can be “passive”, “active” or a combination thereof. In the latter case, the tracking device communicates its location or its received GPS or wireless signals to an intermediary device, such as a docking station, which, in turn, transmits such signals via wired or wireless communications to the remote location. Active tracking devices have the capability of transmitting their location or their received GPS or wireless signals to the remote monitoring center.
Tracking devices can be used in a variety of applications in which persons may attempt to thwart or otherwise interfere with tracking device operation. One such application where this situation arises is where the tracking device along with a remote monitoring center is used to track the location of an “offender”, i.e., an individual who are part of a governmental program, such as parole or the like, wherein monitoring of the offender's location is required. In such applications, the device is affixed to the offender and usually can not be removed by other than authorized persons. Any attempt by the offender or other non-authorized persons to remove the tracking device from the offender or to open the tracking device and disable its operation is detected and results in the transmission of an alarm signal to the remote monitoring station and thereupon appropriate action is taken. While existing tracking devices with tamper detection capability perform satisfactorily, they have certain limitations. For example, when the tracking device is in certain locations, such as being indoors, or in an urban area surrounded by tall buildings, or in a valley surrounded by mountains, hereinafter individually referred to as an “environmentally impaired location,” its ability to receive GPS satellite signals and/or other wireless signals is significantly impaired so as to render the tracking device incapable of providing its normal functions. Moreover, street-savvy individuals have learned that they can mimic this situation by placing a metal foil or the like around the tracking device. At times, this intentional impairment is only for a time period when the offender intends to engage in prohibited activities. During such time period, the location of the offender is unavailable and after removal of the metal foil, the tracking device resumes its normal operation. As a result, there is the unresolved issue as to whether the tracking device was merely in an environmentally impaired location during the time period in which the location of the tracking device is not available or whether there has been tracking device tampering during this period. Moreover, this form of tampering is not limited to offender tracking systems and can also occur in other applications wherein one or more persons desire to thwart the tracking of the monitored entity. For example, some trucking companies that use GPS to track their vehicles have discovered that certain truck drivers wrap the GPS antenna of their truck tracking devices with shielding material to prevent the companies from tracking their truck's location. This above-described shortcoming of tracking devices to provide location tracking renders them incapable of meeting the desired system objectives of certain location tracking applications. Accordingly, it would be desirable if a mechanism could be devised to determine whether there has been tampering or merely a natural loss of signal reception due to the monitored entity being in an environmentally impaired location.